Battery plaques have conventionally been formed by sintering nickel powder onto a coated steel plate. Porosities achieved in the sintered plaque have been generally limited to porosities in the 80% range. These 80% range porosities, in turn, limit the amount of active mass that may be held in the plaque which limits the battery capacity. Low porosity and decreased capacity for holding active mass has long been a problem limiting the battery performance.
Nickel plaques having increased porosity for batteries have been experimentally formed by chemical vapor deposition of nickel carbonyl on carbon felt. The battery plaques were formed by depositing nickel onto a carbon felt substrate and using the nickel coated felt substrate to support active mass. There are several problems with the carbon felt process. First, carbon felt is relatively expensive for the manufacture of batteries; second, the cell size of felt structures varies widely within the felt itself and is difficult to control for fibrous, felt-type structures; third, the carbon felt substrate remains in the battery; and fourth, the process was not satisfactory for polymer coated felts. Previous experimental attempts at chemical vapor deposition of polymer fibers for batteries produced a product having an inferior nickel coating having inferior mechanical stability which was unsuitable for battery plaques.
Recently, in an attempt to overcome the low porosity problem, nickel battery plaques have been produced by an alternative electrochemical method (Matsamoto U.S. Pat. No. 4,251,603). Nickel is electroplated onto a polyurethane foam and sintered to form nickel foam. Before plating may be conducted, polyurethane foam is made conductive by immersion of the foam into a colloidal graphite dispersion and drying the foam. This nickel foam has increased porosity for increasing the amount of active mass battery plaques can support.
Nickel foam, formed by electrochemical technique, has been produced by Sumitomo Electric Industries under the name CELMET.TM. and by SORAPEC under the name METAPORE.TM.. The CELMET.TM. nickel foam has a highly irregular surface when magnified about 100 times. The electrical conductivity of the electrodeposited nickel foam is lower than the expected conductivity as a function of porosity due to the intrinsic structure of the electroplated nickel layer. The poorer conductivity effects battery output, recharging rates and battery overheating during recharging.
Additionally, electrochemically plated nickel foam had less than ideal mechanical properties at high porosities. These lower mechanical properties at higher porosities limit the amount of active mass that may be reliably used in a battery without premature battery failure. A battery plaque formed with electrochemical nickel foam having too high a porosity would cause the plaque to have weak mechanical properties.
It is an object of this invention to produce a nickel foam having improved conductivity.
It is a further object of this invention to produce a nickel foam having improved mechanical properties at higher porosity levels.
It is a further object of this invention to produce a nickel foam having a smaller pore size and more uniform structure for improved battery performance.
It is a further object of this invention to provide an effective method of forming nickel foams with the above improved properties.